1. Field of the Invention
The present invention relates to an image-reading device, and particularly to an image-reading device employing a contact image sensor (CIS) having a plurality of image sensor integrated circuits (IC) chips arranged linearly.
2. Description of Related Art
A conventional image-reading device such as that disclosed in Japanese unexamined patent application publication No. SHO-63-20952 includes a contact image sensor having a plurality of image sensor IC chips arranged linearly. In the example shown in FIG. 1, trigger signals TG1, TG2, and TG3 are inputted simultaneously into image sensor IC chips CH1, CH2, and CH3 so that image signals AO1, AO2, and AO3 are read in parallel from the image sensor IC chips CH1, CH2, and CH3.
One conventional image-reading device disclosed in Japanese unexamined patent application publication No. 2003-298813 includes a contact image sensor having a plurality of image sensor IC chips arranged linearly and divided into blocks of a natural multiple of three. Each block outputs an image signal to a triple-channel analog front end (AFE), thereby improving the speed for reading image signals. The triple-channel AFE is widely used in image-reading devices because, along with single-channel AFEs, triple-channel AFEs are more popular than AFEs having another number of channels and are mass-produced and, therefore, less expensive.
However, this conventional technology can only read an image at one speed, even when the original document is of a narrower width that does not require all image sensor IC chips. Accordingly, the time required to read an image with this conventional image-reading device is governed by the number of pixels in each image sensor IC chip if all chips have the same number, or by the number of pixels in the image sensor IC chip having the largest number when the number of pixels is not uniform.